Saturday, May 21, 2011

Seeing ground clutter on radar

This evening there are severe weather watches across several parts of the central US, including a tornado watch across eastern Oklahoma. Early this evening, a few storms fired to the east of the I-35 corridor--including a couple tornadic ones. Here's the radar image from 0103Z. I noted the locations of the tornadic storms with white arrows.

Now, the large spike off to the southeast of the radar is NOT a sun spike like I talked about the other day. It's actually a blockage return from a large water tower that sits just to the southeast of the radar (this is the KOUN testbed radar in Norman). Most of the radar energy emitted in that direction bounces off the water tower and comes right back to the radar, which the radar interprets as continuous returns from all ranges in that direction. But I want to focus on all of those returns to the west of the radar. It looks like a whole bunch of storms are firing up to the west. But...there's actually nothing there.

So what are those returns? It's something called ground clutter. Here's what happens. Sometimes the vertical density profile of the air in the lower atmosphere arranges itself in such a way that radar beams emitted into the air get bent in odd directions. With the right conditions, sometimes the beams can get bent into the ground.

When an emitted radar pulse hits the ground, it bounces back and returns mostly along the same path upon which it traveled out. As such, the radar interprets this as a very strong return at the range where the radar beam hit the ground. This sort of phenomenon is more common in coastal areas and also when there are strong inversions in the low levels of the atmosphere. So all of those chaotic (but still strong looking!) returns west of the radar are actually ground clutter--areas where the lowest radar beam is hitting the ground.

If you spend a lot of time looking at radar images, ground clutter is often easy to spot. For one, it moves chaotically and not uniformly like precipitation does, and often wildly fluctuates in the strength of the returns. So seeing all these random strong returns dance around in a radar animation is usually strong evidence that you're seeing ground clutter. But there's another way to check for ground clutter--look at the accompanying radar velocity image. Here's the velocity image from the same time as the reflectivity image above.

Fig 2 -- 0.5 degree base velocity from KOUN at 0103Z, May 22, 2011.

Notice how the storms to the east have very nice, colorful velocity fields. You can even see some couplets in the velocity images indicating rotation in those storms. But what about the velocities from all the ground clutter out to the west? They're all gray--indicating zero velocity!

That makes perfect sense, though. There is no Doppler shift when the radar beam hits the ground. After all--the ground isn't moving, so why would it have a velocity value? Any strong thunderstorms are most certainly going to have some pretty strong velocities associated with them. So strong radar returns with zero velocities associated with them are almost certainly not precipitation. They're almost always some sort of clutter.

In fact, the NEXRAD radars have built in algorithms that look for just this combination--strong reflectivity returns in areas of zero velocity--to remove clutter from the radar. So, it's not as common to see this kind of clutter on radars as it used to be. However, the radar I'm showing in the images above (KOUN) is an experimental testbed radar used by the Radar Operations Center, so sometimes they don't have all the algorithms enabled. As such, it's kind of fun to watch it once in a while to see all these random radar phenomena that we never get to see on other radars.

If the right environmental conditions persist, ground clutter can also hang around for a while. Here's the reflectivity image from half an hour later:

Those two tornadic storms that were to the southeast of the radar have actually merged into one storm--which has an even more pronounced hook echo than either of the two storms had before. The dynamics of merging supercell thunderstorms are not well understood and that's an area of ongoing research. In the meantime, you can see some kind of a weak boundary running roughly north to south-southwest just west of the radar. You can often see boundaries between different air masses on radar--but that's a subject for another blog. All those now-weaker returns to the west of the radar are still ground clutter. Often the patterns of ground clutter will take the shape of the local topography, following hilltops and ridgelines (since those are the first places to get hit by the radar beam). Looking at the corresponding velocity image...

Fig 4 -- 0.5 degree base velocity from KOUN at 0133Z, May 22, 2011.

The strong velocity couplet in the tornadic supercell is pretty obvious in this image. But, as expected, the velocities are still zero in the ground clutter to the west. So, I'm afraid it looks like no storms for Norman tonight...

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Completed graduate school at the University of Washington, now a postdoctoral researcher at NCAR in Boulder. The thoughts and opinions expressed on this blog are solely those of the author and are do not necessarily represent the positions of NCAR.
Email me at lukemweather@gmail.com.
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